package a_java_48_day.day6;

/**
 * 不要二
 */

import java.util.Scanner;
public class Main1 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int h=in.nextInt();
        int w=in.nextInt();
        int h12Num=(w/4)*2+(w%4<2?w%4:2);
        int h34Num=(w/4)*2+(w%4<=2?0:1);
        int ans=h/4*(h12Num+h34Num)*2;
        if(h%4==1){
            ans+=h12Num;
        }else if(h%4==2){
            ans+=h12Num*2;
        }else if(h%4==3){
            ans+=h12Num*2+h34Num;
        }else{

        }
        System.out.println(ans);
    }
}